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Chapter 11

Earthquakes

What is an earthquake?

� Earthquake: vibration of Earth produced by the rapid release of energy� Released energy radiates in all directions

from source as WAVES• Epicenter: location of earthquake mapped

on Earth’s surface• Focus = source (at depth) of earthquake

Earthquake focus vs. epicenter

Earthquake Location

� Earthquakes and faults� Faults: large fractures in Earth’s crust � Earthquakes usually associated with

plate movements along faults

� Can be explained by plate tectonics theory

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Earthquake Mechanism

� Elastic Rebound� Rocks on both sides of a fault are

deformed by tectonic forces (strain) � Rocks bend/compress and store elastic

energy • Frictional resistance prevents rock from

slipping

� Frictional resistance is eventually overcome by buildup of strain

Earthquake Mechanism

� Elastic rebound = Earthquake mechanism• Slippage at the weakest point (the focus)

occurs

• Deformed rock “springs back” to its original shape (elastic rebound)

• Vibrations in the form of waves propagate from source

� Earthquakes occur whenever the frictional forces along the fault surface are overcome

Elastic Rebound

Small Earthquakes

� Foreshocks and aftershocks

� Aftershocks: Adjustments that follow a major earthquake often generate smaller earthquakes

� Small pre-earthquakes, called foreshocks, often precede a major earthquake

• By days or, in some cases, by as much as several years

• Foreshocks are unpredictable in their frequency and can not predict major earthquakes

San Andreas: An active earthquake zone

� San Andreas is the most studied fault system in the world

� A transform fault: boundary between the North American plate and the Pacific Plate

� Displacement occurs along discrete segments 100 to 200 kilometers long

• Some portions exhibit slow, gradual displacement known as fault creep (2-4 cm per year)

• Other segments regularly slip larger distances producing larger earthquakes

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The San Andreas fault is part of the boundary between the North American and Pacific tectonic plates. It is classified as a right lateral transform fault.

(To a person facing the fault, the far side shifts to the right.)

“Sag Ponds” within San Andreas Fault trace

Near Parkfield, CA

Offset Streams

San Andreas Fault, CA

Wallace Creek fault investigation area on the Carrizo Plain. Numbers show signed posted nature trail features.

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Offset produced by the 1906 San Francisco earthquake

San Andreas: An active earthquake zone

� Other segments store elastic energy for hundreds of years before rupturing in catastrophic earthquakes

� Great amount of energy stored• Process described as stick-slip motion• Source of great earthquakes• Great earthquakes occur about every 50 to

200 years along these sections

Seismology

� Seismology: the study of earthquake waves, dates back almost 2000 years to the Chinese

� Seismographs: instruments that record seismic waves� Record movement of Earth in relation to a

stationary mass (inertia)� Recorded on a rotating drum or magnetic tape� Horizontal or vertical motion measured� Global network

A vertical ground motion seismograph Horizontal Motion

Seismograph

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Types of Seismic Waves

� Body waves: Travel through Earth’s interior

� Two types based on mode of travel

(1) Primary (P) waves: “Pressure” waves• Push-pull (compress and expand) motion• Changes volume of the material• Travel through solids, liquids, and gases• “Pressure” waves• FASTEST type of wave: 1.7 times FASTER

than S waves

Primary (P) Waves

(a.k.a. “pressure waves”)

Types of Seismic Waves

� (2) Secondary (S) waves• “Shake” (sine wave) motion at right angles

to their direction of travel

• Travel only through solids

• Slower than P waves

• Slightly greater amplitude than P waves Secondary (S) Waves

Types of Seismic Waves

� Surface Waves: Travel on Earth’s Surface� Complex motion: can be vertical (rolling

waves) or side-to-side shaking

� Cause greatest destruction of infrastructure and most severe shaking

� Exhibit greatest amplitude (energy) and slowest velocity of 3 types of seismic waves

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Locating the source of earthquakes

� Epicenter is located using the different velocities of P (faster) and S waves (slower)� Measure the time interval between the arrival

of the first P wave and the first S wave at seismograph

� Greater “lag-time” between P and S waves means greater distance from epicenter

� A travel-time graph is used to determine each seismograph station’s distance to the epicenter

Seismogram showing P, S, and surface waves

Locating Earthquakes

Travel-Time Graph

Lag-time vs. Distance from Epicenter

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Locating the source of earthquakes

� Locating the epicenter of an earthquake� Three station recordings are needed to locate

an epicenter

• “Triangulation”� A circle with a radius equal to the distance to

the epicenter is drawn from each station

� The point where all three circles intersect is the earthquake epicenter

Locating Earthquake Epicenters: Triangulation

Earthquake Belts

� 95% of earthquakes originate in a few relatively narrow belts that wind around the globe

� Major earthquake belts � Circum-Pacific belt:

� “Pacific Ring of Fire”

� Mediterranean Sea region to Himalayan complex

� Mid-oceanic ridge system

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EarthquakesMagnitude 3.5

and Greater

1974 - 2003

Earthquake Depths

� Earthquakes originate at depths ranging from 5 to nearly 700 kilometers

� Earthquake foci classified as• Shallow (surface to 70 kilometers)• Intermediate (between 70 and 300

kilometers)

• Deep (over 300 kilometers)

Earthquake Depths

� Definite patterns exist� Shallow focus occur along the oceanic

ridge system� Deep-focus earthquakes occur in the

circum-Pacific belt • Landward of deep-ocean trenches• Subduction-related• Compelling evidence for Plate Tectonics Subduction and Deep-Focus Earthquakes

Measuring Earthquakes

� Two measurements that describe the size of an earthquake are

• Intensity – a measure of the degree of earthquake shaking

• Based on amount of damage• Monetary assessment ($$$)• Variable dependent on geography

• Magnitude – estimates the actual amount of energy released at the source of the earthquake

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Measuring Earthquakes

� Intensity scales (damage-based)

� Modified Mercalli Intensity (MMI) Scale• Developed using California buildings as its

standard• “Shake Maps”

� Drawback of intensity scales:• Destruction may not be a true measure of the

earthquake’s actual severity (energy released)• Population, preparedness, and building codes a

factor

Measuring Earthquakes

� Magnitude scales (energy-based)� Richter magnitude - concept introduced

by Charles Richter in 1935� Richter scale

• Based on amplitude (energy) of the largest seismic wave recorded

• Accounts for the decrease in wave amplitude with increased distance

Measuring Earthquakes

� Richter scale: Logarithimic� Each unit of Richter magnitude increase

corresponds to a tenfold (10X) increase in wave amplitude and a 32-fold energy increase

� Magnitudes less than 2.0 are not felt by humans

� Largest magnitude ever recorded on a seismograph was 9.5 (Chile, 1960)

• USA: 9.0 (Anchorage, AK) • 2004 Sumatra = 9.3

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Earthquake Magnitudes on the Richter Scale

Earthquake magnitudes 2.5 or less are usually not felt, but can be

recorded by a seismograph. There are about 900,000 of these minor

earthquakes each year.

Earthquake magnitudes 2.5 to 5.4 are often felt, but cause little

damage. There are approximately 30,000 such earthquakes every year.

Earthquake magnitudes 5.5 to 6.0 cause slight damage to buildings and

other structures. There are about 500 of them every year.

Earthquake magnitudes 6.1 to 6.9 may cause a lot of damage in very

populated areas. There are about 100 of these strong earthquakes each

year.

Earthquake magnitudes 7.0 to 7.9 are considered major earthquakes

because they always cause serious damage. There are only about 20 such

earthquakes yearly, and the Haiti 2010 earthquake is included in this range.

Earthquake magnitudes 8.0 and higher are great quakes that can totally

destroy a community, and they only occur once every five to ten years.

Earthquake destruction

� Ground shaking� Shaking decreases with distance from

epicenter

� Destruction varies considerably due to local geology

� Regions within 20 - 50 kilometers of epicenter experience about equal intensity ground shaking

Earthquake destruction

� Liquefaction� Unconsolidated materials saturated with water

turn into a mobile fluid and will FLOW

� Amplifies intensity of ground shaking

� Sand boils or sand volcanoes can occur

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Earthquake destruction

� Structural damage depends on� Intensity and duration of the shaking� Nature of the geologic material upon

which the structure rests• Liquefaction

� Design of the structure

Liquefaction Hazard Map: Oakland, CA

Cypress Viaduct DamageLoma Prieta Earthquake Oct. ‘89

Cypress Viaduct DamageLoma Prieta Earthquake Oct. ‘89

Liquefaction DamageLoma Prieta Earthquake Oct. ‘89

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1964 Anchorage, AlaskaGood Friday Earthquake

Liquefaction: Turnagain Heights

Anchorage, Alaska: 1964 Good Friday Earthquake

Turnagain Heights, AK Turnagain Heights, AK

Anchorage, AK 1964 Liquefaction: Niigata 1964

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Sand Volcanoes

(Sand Boils)

Evidence for Liquefaction

Earthquake destruction

� Tsunamis, or seismic sea waves� Destructive oceanic waves that are often

called “tidal waves” (inappropriately)� Caused by either

• vertical displacement along a fault on the ocean floor or

• a large undersea landslide triggered by an earthquake

Earthquake destruction

� Tsunamis, or seismic sea waves� In the open ocean height is usually < 1

meter� May travel > 600 mph � In shallower coastal waters the water

piles up to heights that occasionally exceed 30 meters

� Can be very destructive

Formation of a tsunamiFormation of a tsunami

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Anchorage, AK 1964 12/26/2004 Indian Ocean Tsunami

12/26/2004 Indian Ocean Tsunami

Earthquake Prediction

� Short-range predictions

� Goal: provide a warning of the location and magnitude of a large earthquake

� Possible precursors – phenomena that precede a forthcoming earthquake • measuring uplift, tilt, or subsidence

• strain in the rocks

Earthquake Prediction

� Short-range predictions� No reliable method exists for making short-

range earthquake predictions � Long-range forecasts

� Give the probability of a certain magnitude earthquake occurring within 30 to 100 years

� Based on the premise that earthquakes are repetitive or cyclical

• Using historical records or paleoseismology

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